There is an increasing demand for an increase in strength and a reduction in weight of automotive hot rolled steel sheets from the viewpoint of the improvement in fuel efficiency linked to improvements in automobile safety and protection of the environment. Particularly, the mass of frames and arms, which are referred to as underbody systems among automotive parts, constitute a large part of the total mass of a vehicle. So, a material which is used for such parts is processed to have an increased strength and thereby to be thinner, and in this manner, the reduction in vehicle weight can be achieved. Moreover, the material which is used for the underbody systems requires high fatigue properties from the viewpoint of durability relating to the oscillation while the vehicle is running, and thus high-strength steel sheets are widely used. Among them, hot rolled steel sheets are mainly used for the reason of price advantage.
Among the hot rolled steel sheets, there are known low-yield ratio DP steel sheets in which a ferrite phase and a martensite phase are combined and TRIP steel sheets in which a ferrite phase and an (remaining) austenitic phase are combined as steel sheets having both a high strength and good workability and formability. However, while these steel sheets have a high strength and excellent workability and ductility, they cannot be said to have excellent hole expansibility, that is, stretch-flange formability. In the case of constituent parts requiring stretch-flange formability, such as underbody parts, bainite steel sheets are generally used despite their slightly poor ductility.
A considerable reason for the poor stretch-flange formability of composite structure steel sheets such as a composite structure steel sheet (hereinafter, may be described as “DP steel sheet”) formed from a ferrite phase and a martensite phase is that, since the composite structure steel sheets are composite bodies of a soft ferrite phase and a hard martensite phase, stress is concentrated at the boundary parts between both the phases in hole expansion and the boundary parts thus easily become the starting points of fracture due to failing to follow deformations.
In order to overcome the problem, there are proposed several steel sheets, in which a DP steel sheet serves as a base for the purpose of achieving both mechanical strength characteristics and fatigue properties or hole expansibility (workability). For example, in Patent Document 1, as a technique for the stress relaxation achieved by dispersed fine grains, a steel sheet is disclosed in which fine Cu is precipitated or solid solution is dispersed in a composite structure steel sheet (DP steel sheet) formed from a ferrite phase and a martensite phase. In the technique shown in Patent Document 1, it is found that, the solid-soluted Cu or the Cu precipitate in which the size of grains consisting of only Cu is equal to or smaller than 2 nm is very effective in improving fatigue properties and also does not damage workability, and then, composition ratios of various compositions are limited.
In addition, as a technique for the stress relaxation achieved by reducing a strength difference between combined phases, for example, Patent Document 2 discloses a technique in which by reducing the content of C as much as possible, a bainite structure is employed for a main phase and a ferrite structure subjected to solid solution strengthening or precipitation strengthening is included at a proper volume ratio to reduce the difference in hardness between the ferrite and the bainite and to avoid the generation of coarse carbides.